IRIS

Intelligent Roadway Information System


Travel Time Estimation

The current travel time from a DMS to a downstream station can be estimated and displayed on the sign. A new estimate is made every 30 seconds, based on speed data from vehicle detection systems.

Route Pathfinding

This feature requires that the road topology is configured for the entire route. Using this topology, the shortest route is found from the DMS to the destination station.

A route consists of one or more corridor trips. Each corridor trip has an origin and a destination, which are r_nodes associated with a single freeway corridor. For the first corridor trip, the origin is the DMS location. A route can only branch if an r_node with exit node type is matched with an entrance r_node. In order to match, the roadway / direction for the exit r_node must equal the cross street / direction for the entrance, and vice versa.

Two system attributes control route pathfinding. route_max_legs is the maximum number of corridors to branch. route_max_miles is the maximum total route distance.

Speed Smoothing

To deal with sampling problems at low speeds, a smoothing procedure is used. For each station, running average and running minimum speeds are calculated from all valid detectors in the station. If a speed higher than the speed limit is recorded, the speed limit is used instead.

The sample interval used increases as the speed becomes lower:

Route Travel Time

The distance between each consecutive valid station is divided into 3 links of equal length. If any link is longer than 0.6 miles, a travel time estimate would not be reliable, so the process is aborted. The links which are adjacent to a station are assigned speed from that station. “Middle” links are assigned an average of the immediate upstream and downstream station speeds.

Normally, the running average speed is used. If the link is less than 1 mile from the corridor destination, the running minimum is used instead. Travel time for each link is estimated by dividing its length by the assigned speed. If the route branches, each branch adds a 1 minute turning delay.

Travel Time Limit

To prevent unreasonable messages from being displayed due to data errors, a limit is calculated. The total route length is divided by the value of the travel_time_min_mph system attribute. The result, rounded up the to next 5 minutes, is the travel time limit.

If the calculated travel time is over the travel time limit, the message will be over limit. If the over limit mode is blank, no message will be shown. Otherwise, the tag will be replaced with the travel time limit, with the over limit text either prepended or appended.

Travel Time Action Tag

The estimated travel time (in minutes) can be displayed in DMS messages using DMS actions. A [tt ] action tag in the quick message will be replaced with the appropriate value. It has the following format:

[tt dest,mode,over ]

Parameters

  1. dest: Destination station ID
  2. mode: Over limit mode (prepend if omitted)
    • blank: do not display travel time when over limit
    • prepend: prepend over limit text before travel time
    • append: append over limit text after travel time
  3. over: Over limit text (OVER if omitted)

NOTE: multiple destinations can be included on the same message by specifying multiple [tt…] tags.

Examples

FREEWAY TIME TO[nl][jl2]I-94[jl4][ttS100,prepend,OVER ] MIN
TIME TO[nl][jl2]I-35W[jl4][ttS200,append,+] MIN
DOWNTN[nl][ttS300,blank] MIN